How should I tackle this breechplug?

[45dash100]

Hello everybody.  I "started" building a flintlock over 10 years ago, and didn't make much progress.  Finally back at it, and the breechplug install has got me scratching my head.  Most tutorials and pictures I can find on this subject, have a much larger sealing area to deal with.

I'm wondering how it'l even be possible to get a good seal, when there's so little size difference between the plug threads, and the bore.  At least by hand.

Specs first.

This is a Green Mountain 15/16 - 54 caliber barrel.
Best I can measure with calipers, the bore diameter is .535", and the groove depth is .010 (groove diameter of .555).
Barrel is tapped 5/8-18 UNF.

Here's the best real world picture I could get.  Need 3 hands to get something clearer.  There's some prussian blue in there, I was checking to see what the breechplug would do.  As it stands, the tip of the plug doesn't contact the sealing surface, even though the plug depth is longer than the barrel hole is deep.




I believe the threads in the barrel end before the rifling starts.  Drew things up in CAD since it's a good way for me to wrap my head around how things work.  The distance between the end of the threads and the start of the rifling is exaggerated.






So, I believe, that in addition to getting the tang to match up with the correct flat (opposite the Green Mountain stamp on the barrel), and getting the plug tip to bottom out and seal, I will need to carefully remove threads from the end of the plug so it'l even screw in far enough.

Is that how you guys would tackle this?  Or would it be better to thread the barrel a little deeper?  Wondering if I shouldn't just send this off to someone with a lathe.

Thanks!

[Bob Roller]

That hole was simply drilled with a common drill bit and the area shown in blur is the angle established by the drill.
It can be a trap for fouling but not dangerous to shooting.Fixing it would involve a lathe and 2 minutes of time but then a longer plug would be needed to have the seal you want.
Bob Roller

[45dash100]

That hole was simply drilled with a common drill bit and the area shown in blur is the angle established by the drill.
It can be a trap for fouling but not dangerous to shooting.Fixing it would involve a lathe and 2 minutes of time but then a longer plug would be needed to have the seal you want.
Bob Roller

Thanks for the reply!  I could have sworn the edge where the rifling starts was flat when looking at it with the naked eye.  Figured I better get some better real pictures, not just my cad drawings.

Found a flashlight, and with some creative use of masking tape, was able to get a better picture.  Here it is with the Prussian blue stuff cleaned out.  Got a shot where you can see the threads ending.



I think it must be machined flat.  It doesn't seem like the edges would look so clean, especially when you look at the sharp corner left near the grooves.  Maybe a really shallow angled tipped drill bit?  I'm definitely not an machinist though, so it's all speculation on my part.  You can also see some scuffing, and perhaps a slight bevel on the edge of one of the lands.  Wondering if that might have been from me trying to seat a plug forever ago.

Either way, it probably needs some work to get a good seal.   The only reason I'm concerned about a good seal, is all the warnings I see online about corrosion buildup eventually making the gun unsafe, even if it didn't start out that way.  Am I worried about nothing with as small as the gaps will end up being here?

Know anybody I could send this off to?  Could the regular plug still be used if the threaded section of the barrel was shortened a bit?  The breech plug I have (Track of the Wolf Part #FT-16-5) has a depth of .626, and the threaded barrel section, a depth of .616.  Not sure how many threads are needed to be safe, but I see people throwing around ballpark numbers like .5" is good.

[Bill Raby]

All you need to do is file the breech plug a little shorter until you get a tight fit. Then file a little more off, along with a matching amount from the back of the barrel until the flats line up. Only need  a file and it will not take very long.

[Bob Roller]

Carefully use a file with safe edges and fie the plug ONLY where it contacts the barrel.Tighten and see if it lines up with a flat and hope you haven't established a"top flat"with a dove tail or sights.The safe edged file will protect the plug'd threads.
Bob Roller

[45dash100]

All you need to do is file the breech plug a little shorter until you get a tight fit. Then file a little more off, along with a matching amount from the back of the barrel until the flats line up. Only need  a file and it will not take very long.

Problem is, the threads in the barrel don't go deep enough, and the breech plug bottoms out against the threads before contacting the rifled section of the bore.  I think I'm going to need to file the plug down, and also remove threads from the tip of the plug, so it'l actually screw in far enough to get good contact.

Here's another picture, again gaps are a bit exaggerated, I don't think the front of the plug is actually that far from the rifling, and the gap at the back is also smaller. It's just easier to see the problem like this.


Out of threads, plug can't be screwed in any further:

Carefully use a file with safe edges and fie the plug ONLY where it contacts the barrel.Tighten and see if it lines up with a flat and hope you haven't established a"top flat"with a dove tail or sights.The safe edged file will protect the plug'd threads.
Bob Roller

The barrel exterior is clean fortunately, no dovetails or sights cut into it yet.  Only thing on the exterior is the Green Mountain Logo, which I'm led to believe needs to be the bottom flat due to potential run-out.

[Bob Roller]

A sharp flat file wide enough to cover the plug and a machinists right angle gauge,a small one.Also a magnet to capture the filings.
Bob Roller

[45dash100]

I can file the end of the plug, or the barrel down without too much trouble.  The part that has had me concerned from the beginning, is removing threads from the plug so it'l seat deep enough.  And doing so without making a mess of the taper on the front of the plug.  There is so little seating area that if any of this is going to be worth it, it needs to be near perfect.

Saw a post that disappeared mentioning putting the plug into a drill to make even filing easier.  I don't have anything big enough to do that, but I can easily 3D print a jig.

I've got some 3/8" all thread, and a few 8-32 screws lying around.  So I'm just gonna print out the green part.  A few hours print time, and about a dollar of plastic should get me something workable.  In the meantime, I'll go file on horrible sand cast parts that came with the kit.

[Bob Roller]

The same effect can be has with 2 nuts jammed together and tightened in opposite directions.That IS a good looking design there.

Bob Roller

[45dash100]

The same effect can be has with 2 nuts jammed together and tightened in opposite directions.That IS a good looking design there.

Bob Roller

Yeah, thought about trying that, or just using a bolt.  Not quite enough clearance, unless I went down a size from 3/8".  Fortunately, the drill press spins the right way, so it shouldn't be an issue, the threads will hold it in place.






Should work well I'm hoping.  Will probably print out another plastic nut to serve as a guide and thread protector for the plug.

Will use this to file a slight bevel onto the threads until it screws far enough in to contact the rifled shelf.  Once that proof of concept works out, I can file the plug and barrel both until things line up like I want.

[RAT]

Take a 5/8-18 tap and grind the surface to make a modified bottom tap. Slowly and gently run it into the hole to finish cutting that last bit of thread in the barrel. Use lots of oil. Clean it and re-check your fit.

Generally speaking... It still won't go all the way down because the plug itself has a lead angle at the end that doesn't match the threads in the barrel. Adjust this by filing the angle at the end of the plug until the flat face of the plug makes solid contact with the shoulder inside the barrel. Since that shoulder inside the barrel isn't very wide, go slow and don't take off too much.

Only when the face of the plug is making solid contact with the shoulder can you start filing on the face of the plug to shorten it. I take a hardware store nut and thread it on to help me file the face flat. Screw the nut on and black the face and end of the nut with marker. That tells you if you're filing it flat. File a few strokes... check the fit... file a few more strokes... check the fit again... and so on. It's pretty tedious, but it works.

When you get the face to make contact, and the tang stops a little short of lining up with the desired flat... say the tang is about 1/32" away from lining up... remove the plug, put some anti-seize grease on it, and screw it back in tight. Give it a little nudge with the wrench and see if it will move that last 1/32".

[Joe Stein]

I agree with Bob/RAT.  I think somebody didn't use a bottoming tap when the barrel was threaded initially. The easiest way to make a bottoming tap is to take a regular plug tap and cut it off with a cut-off wheel in a Dremel. Cut it where the thread diameter is full thickness.  Clamp the square end in a vise and keep checking the temperature of the tap. When you feel the tap get hot enough that you can't hold a finger on the place where you are cutting, dunk it in water to cool it off, then continue cutting.
It takes a fair amount of power to turn a 5/8-18 tap, so use a good thread cutting fluid, and don't be afraid if it seems hard to turn the tap wrench.
Once you have a good thread all the way down, continue as Bob explained.

[45dash100]

I agree with Bob/RAT.  I think somebody didn't use a bottoming tap when the barrel was threaded initially. The easiest way to make a bottoming tap is to take a regular plug tap and cut it off with a cut-off wheel in a Dremel. Cut it where the thread diameter is full thickness.  Clamp the square end in a vise and keep checking the temperature of the tap. When you feel the tap get hot enough that you can't hold a finger on the place where you are cutting, dunk it in water to cool it off, then continue cutting.
It takes a fair amount of power to turn a 5/8-18 tap, so use a good thread cutting fluid, and don't be afraid if it seems hard to turn the tap wrench.
Once you have a good thread all the way down, continue as Bob explained.

Yeah, I ordered a tap this morning.  Good suggestions from everybody.  Didn't want to mess with a tap if I didn't need to, but it's far enough away that I think it's warranted.  Oh well, won't slow me down anyway.  Can't get much slower than the old worn out files I'm using to work on the barrel.  Ordered a new good set a month ago, and they're still not here. So, I've ordered a big double cut which will hopefully be here next week.

Got a filing guide all ready to go.



I think filing on the barrel will be much easier as far as keeping things square.  The plug has two separate faces, which looks rather difficult to keep even.

[Eric Krewson]

I have run into this several times, being simple minded I just file most off the last thread off the plug and fit it in where it needs to be. No fancy jigs, I do it by hand. Seems like GM breeches are 5/8" deep which throws off the lock fence position, 1/2" deep lines everything up so I shorten the breech first. I do spend a lot of time hand fitting a plug to get a good seal, I spent all day once, file a little, fit a little until I had it just right.

Of course, I am just a beginner wood hack and don't have the machinist skills like many here so I have to improvise.

[satwel]

Take a 5/8-18 tap and grind the surface to make a modified bottom tap. Slowly and gently run it into the hole to finish cutting that last bit of thread in the barrel. Use lots of oil. Clean it and re-check your fit.

Generally speaking... It still won't go all the way down because the plug itself has a lead angle at the end that doesn't match the threads in the barrel. Adjust this by filing the angle at the end of the plug until the flat face of the plug makes solid contact with the shoulder inside the barrel. Since that shoulder inside the barrel isn't very wide, go slow and don't take off too much.

Only when the face of the plug is making solid contact with the shoulder can you start filing on the face of the plug to shorten it. I take a hardware store nut and thread it on to help me file the face flat. Screw the nut on and black the face and end of the nut with marker. That tells you if you're filing it flat. File a few strokes... check the fit... file a few more strokes... check the fit again... and so on. It's pretty tedious, but it works.

When you get the face to make contact, and the tang stops a little short of lining up with the desired flat... say the tang is about 1/32" away from lining up... remove the plug, put some anti-seize grease on it, and screw it back in tight. Give it a little nudge with the wrench and see if it will move that last 1/32".

This is the correct answer to your problem. First, you need to cut the threads a little deeper until they reach the start of the rifling. I've had to do this with two barrels. I'm not sure if I read it on line or in their old printed catalog, but Track of the Wolf has instructions on how to perform this necessary step. I ground the tapered section off of my taps using a bench grinder and turned them into bottoming taps. You don't need to turn them very far to complete the threading. Use lots of cutting oil. Once the threads reach the rifling, you can start shortening the plug face.

[Scota4570]

I love the CAD drawings and printed devices.  That seems to be the way of the future. 

Breech plug fitting is very routine work that I do on an antique lathe.   The threads are bottomed using a tap with the taper ground off.   If the threaded hole in the barrel is too deep face off the barrel and make it right.  IT is expected for the plug nose to needs fitting.  IT is impossible to thread to the very bottom of a hole.  Cutting a relief with a boring bar can be done but it is easier to fit the plug nose to the hole you got.  The nose of the plug should bottom out in the hole. There should be a shoulder at the bottom of the hole.   At the same time the tang need to fit with contact.  to the barrel.  I do all that with a lathe and a calculator.  The lathe doe a nicer job.  Most guys file and spot. 

Consider the barrel makers breech plug threads to be a rough start. 

IF you can justify it find an old lathe.  I have a Southbend Heavy 10 with lots of accessories including collets.   I use it multiple times a week.  I'd find another hobby if I did not have my lathe and mill.  Being a competent lathe operator is a valuable skill for gun work.  Old lathes are not expensive. 

[Bob Roller]

Good old AMERICAN lathe and good for another 100 years if taken care of.Making a bottoming tap from a plug tap works for me and I have done it many times in my shop and several others.I did not like having to buy a Chinese made cut off saw but American manufacturing was replaced by sandwich shops and aggressive Asians who obviously have an idea of how to make things.I have nothing against sandwich shops but seeing rows of them along the roads and an economy based on hoots and giggles is a vexation while on vacation.
Bob Roller

[45dash100]

I have run into this several times, being simple minded I just file most off the last thread off the plug and fit it in where it needs to be. No fancy jigs, I do it by hand. Seems like GM breeches are 5/8" deep which throws off the lock fence position, 1/2" deep lines everything up so I shorten the breech first. I do spend a lot of time hand fitting a plug to get a good seal, I spent all day once, file a little, fit a little until I had it just right.

Of course, I am just a beginner wood hack and don't have the machinist skills like many here so I have to improvise.

Yeah, if this had been say a .50 cal barrel with the same size breech plug, I wouldn't have even made this thread.  The shelf you need the plug to mate with would be sizeable, and far easier to get right.  With a 54 cal barrel, and this plug diameter, there's so little wiggle room that a single ding or slip of the file would render all the work moot.

I've done some reading that also suggests an excessive plug depth could cause me issues.  I'll have to lay stuff out, and see if I can work it in, or if I would be better off removing more barrel material.  That's a lot to file off.  I'd be really tempted to slowly chop off the bulk of the material with an angle grinder, then do the rest with a file.

I love the CAD drawings and printed devices.  That seems to be the way of the future. 

Breech plug fitting is very routine work that I do on an antique lathe.   The threads are bottomed using a tap with the taper ground off.   If the threaded hole in the barrel is too deep face off the barrel and make it right.  IT is expected for the plug nose to needs fitting.  IT is impossible to thread to the very bottom of a hole.  Cutting a relief with a boring bar can be done but it is easier to fit the plug nose to the hole you got.  The nose of the plug should bottom out in the hole. There should be a shoulder at the bottom of the hole.   At the same time the tang need to fit with contact.  to the barrel.  I do all that with a lathe and a calculator.  The lathe doe a nicer job.  Most guys file and spot. 

Consider the barrel makers breech plug threads to be a rough start. 

IF you can justify it find an old lathe.  I have a Southbend Heavy 10 with lots of accessories including collets.   I use it multiple times a week.  I'd find another hobby if I did not have my lathe and mill.  Being a competent lathe operator is a valuable skill for gun work.  Old lathes are not expensive.

Feels a little weird to combine 3D printing with muzzleloaders, but I figure the barrel manufacturer probably built the barrel with a few CNC machines, so I'm already missing that bit of authenticity.  My little jigs are probably a step down in tech.   

I really wish I could get a lathe.  Only place I have room to stick one, would require bringing in more power than my off grid setup.  It's something future me will have to enjoy.  I'll have to settle for the drill press and jigs for now.

[davec2]

This old thread may be helpful:

https://americanlongrifles.org/forum/index.php?topic=61717.msg619005#msg619005

I wrote the following as part of that thread.......

While I agree with Dan and Scota that one should do the best job possible, one can worry too much about perfection where it is not really needed.  I think JerryWH, and others, have shown pictures of a plethora of original breech plugs that would cause some here to regurgitate on their keyboards.  Nonetheless those plugs had long service lives, didn't rust into oblivion, and didn't blow out to blind or kill the operator.  Now I'm not saying that a sloppy fit is a good idea, but on the other hand, I have read innumerable posts about how imperative it is to "seal" the breech against 10,000 to 20,000 psi hot gasses by tightly butting a flat plug face against a steel shoulder.  In my rocket engine designer and fabricator life I have to ABSOLUTELY seal very hot gasses (i.e.~6,000 degrees F) and at pressures on the same order of magnitude or the engine will destroy itself in milliseconds.  As a consequence, I am familiar with a myriad of ways to get that type of hot gas seal.  I can tell you unequivocally that a flat plug against a stepped shoulder cannot do that job.  Black powder breech plugs appear to seal well for three reasons:  1) The pressure they see is not sustained but builds and then dissipates in milliseconds.  If you were to put 10,000 psi in the bore and keep the pressure that high while you leak checked the breech plug with a soap solution, it would be blowing bubbles like a two year old at a birthday party. 2) After the first shot, some powder particulate residue will pack into whatever little gas path is available and it makes a good seal itself.  3)  The gas that gets past the not very efficient (even if very well done) face seal on the plug has to get past all the threads to make itself apparent.  Each time the gas goes around a thread, it dissipates in pressure as it is forced to turn corners.  In rocket engine turbo pumps we do this on purpose with something called a "labyrinth seal".  With only an instantaneous driving pressure in the bore, and unless the breech plug fit is grossly poor, the gas will never make it out at the tang end of the plug.

All that said, my over wordy point is, do the best job you can fitting a plug, but it doesn't have to be perfect (for either performance or safety reasons).  As Hungry Horse says the old time plugs worked fine and were far, far less precise than anything the barrel makers of today produce.  Life is way too short to spend hours or days on a level of precision not required to do the job.

Just my thoughts

PS:  Get the fit close and then, if you're worried about "powder residue" and / or corrosion, use some of the removable thread locker on the plug when you put it together finally.....or just lube it with an anti seize grease.

[okawbow]

I completely agree with Dave2. The only thing between you and the explosion in your barrel are the breech plug threads. The only benefit of getting a “seal” between the flat end of the breechplug and the shoulder of the threaded hole, is to help reduce the fouling ring at that connection. The threads do the real sealing and maintain the strength of the joint. A plug that is tightened too hard against the shoulder is actually weaker, because the threads are stressed. The best compromise is that the end of the breech plug just contacts the shoulder and the end of the barrel at the same time. I feel the breech plug should bottom out with normal pressure from an 8” adjustable wrench.

[stan57]

Ehhh, decided to delete my wordy newb comment. Y'all continue.

[45dash100]

All that said, my over wordy point is, do the best job you can fitting a plug, but it doesn't have to be perfect (for either performance or safety reasons).  As Hungry Horse says the old time plugs worked fine and were far, far less precise than anything the barrel makers of today produce.  Life is way too short to spend hours or days on a level of precision not required to do the job.

Just my thoughts

PS:  Get the fit close and then, if you're worried about "powder residue" and / or corrosion, use some of the removable thread locker on the plug when you put it together finally.....or just lube it with an anti seize grease.

Heh, it was gonna take hours no matter what.    I don't use hand files much, so making sure I file things flat has been tricky, and needing to line up with a particular flat made it that much worse.  Good to know it won't blow up, even if things aren't perfect.

I probably spent more time shaping the tang, it wasn't machined straight in any dimension, and a lot of material had to be removed.

Bottom tap got here at the end of the week, and made things a lot easier.  Took nearly an additional full turn of the tap to get the threads deep enough.



Here it is installed with some antiseize.  Plug fit against the shelf was as perfect as I could get it.  Glad I won't have a water or fouling trap.

[Bob Roller]

Having the right tool IS a big convenience and there's no way around it.
Bob Roller

[ScottH]

That turned out looking good!

[Long John]

45,

I am very impressed with your attention to detail and your use of additive manufacturing methods to make your jig.  I am doing a breech-plug fitting demo at Kempton this coming weekend.  It is scheduled for Sunday morning.  I use just hand tools, no lathe, and it takes about an hour to do.

I must take issue with a couple of statements made in this thread, straight threads do not seal, there is always a minute gap between the male and female threads on straight threads.  The objective of the shoulder is to seal against a perpendicular face of the breech plug,  Since relatively soft metals are used for both the two surfaces will deform slightly as the plug is tightened up and as long as there are no burs or other irregularities, the seam between the two parts will be adequate for the momentary pressurization at black powder pressures.

Good luck with your rifle build.  this stuff is fun, isn't it?

JMC